A serial interconnection between separate system modules provides the simplest communication network between these modules. Due to its simplicity, this type of network is useful in practice for cost and reliability reasons. In general, a system module can be any subsystem, as simple as a single passive component or as complicated as a Phased-Locked Loop (PLL), an entire radio, an antenna phased array, or some other complex circuit. A serial interconnection uses a transmission medium with unidimensional signal propagation properties such as an electrical cable, an optical fiber, a strip line, a microstrip line, a coplanar line, a wireless narrow beam, etc. The system modules connected serially attach to this transmission medium and receive or transmit signals, usually according to a protocol. For example, a simple protocol involves distributing a signal from one module to all other modules. A more sophisticated protocol might involve both transmitting and receiving signals from one module to any other module or set of modules.
In many applications, it is important to know precisely the time delay a signal undergoes when propagating over the serial interconnection from one module to another module. For example, if we distribute serially an LO (local oscillator) or a sampling signal over a phased array containing many radio modules, it is important to correct the phase differences between the signals received at each radio module due to propagating delays. Without this correction, also referred to as “phase calibration”, the proper functionality of the phased array would be compromised since the very operation of a phased array relies on precise global phase alignment of signals at all radios.
Similarly, in some applications not only time delays or phase shifts but also signal magnitude changes due to transmission loses or other effects must be corrected. For example, the Intermediate Frequency (IF) lines in the active arrays described in U.S. Pat. No. 8,611,959, filed Dec. 17, 2013, all of which is incorporated herein by reference, have practical losses in addition to phase shifts. These losses need to be compensated for correct system operation. The magnitude correction is also called “magnitude calibration”.
In production samples, time delays producing phase shifts and magnitude changes due to signal transport over a serial interconnection can be calculated or directly measured. However, such methods can be used for phase/magnitude calibration only if the physical implementation of the serial interconnection has transmission properties which are predictable to the desired precision after manufacturing and do not vary unpredictably over expected changes in operating conditions, such as temperature and humidity. If, for example, the transmission properties of the serial interconnection are subject to manufacturing variations beyond the desired precision, any pre-production calculations and simulations or any direct measurements of production samples cannot represent correctly the transmission properties of all units produced.
Likewise, even if all production units have predictable transmission properties at factory temperature and humidity conditions, these properties might vary unpredictably beyond the desired precision with field operating conditions. In such cases, the methods described above for identifying time delays and magnitude changes of serial interconnects cannot be used to compensate correctly the time delays and magnitude changes in the field.
When phase/magnitude calibration of a serial interconnection is required, the usual practice is to fabricate the serial interconnection with materials and design techniques ensuring predictable characteristics over manufacturing and operating conditions. This comes with a significant cost penalty in most cases. Take, for example, a phased array, which is a large electrical system, i.e., a system with physical dimensions large compared to the wavelength of the operating frequency. If high frequency signals such as LO (local oscillator) signals propagate through the phased array over a serial interconnection, very large phase skews occur (e.g., thousands of degrees) yet the compensation of these skews (phase calibration) must reduce them to only a few degrees. This cannot be accomplished unless the natural skews are predictable to this level of precision. In order to fabricate transmission lines with such accurate characteristics, expensive materials (e.g. dielectrics, etc.) and high fabrication tolerances (e.g. line widths, thicknesses, etc.) are required.
A low cost method for designing serial interconnections with inherent phase calibration is described in U.S. Pat. No. 8,259,884, filed Jul. 21, 2008, all of which is incorporated herein by reference. Other approaches to the same effect are also described in prior art identified in U.S. Pat. No. 8,259,884. In these methods, rather than relying on expensive materials and fabrication tolerances, the design relies on mutual compensation between signals propagating over matched transmission lines. These methods further rely on various high precision analog circuits. In practice, these analog circuits are challenging to design and are difficult to scale or port from one implementation to another because they require operation at high speed and with high precision simultaneously.